In the manufacture of integrated circuit devices, it is customary to do most of the processing of the devices on a wafer scale in a large wafer, typically many inches in diameter, and after such processing to dive the wafer into individual chips, each typically a square or rectangle in shape and a fraction of an inch on a side, for completion of the final steps of the processing, such as the provision of terminal connections and packaging.
Basic to the processing, is the water use of a step and repeat printer to print, in a layer of photoresist, the appropriate patterns of the various photoresist masks that are used to localize the effect of the many processing steps.
Generally the shape of the wafer being processed is circular, while the shape of the chips after dicing is essentially square or rectangular. Accordingly the area generally printed in each step generally is either a square or a rectangle. It will be convenient in this description to describe the area printed in each step as a square, although it is to be understood that this term as used here in, includes rectangular areas. Because the wafer is circular and the areas exposed by each step are square, there will be a width around the periphery (edge) of the wafer that, if printed, would yield incomplete squares that would be useless as chips. Additionally, there is a region around the wafer edge that is also useless because of a high density there of various forms of defects.
Moreover, presently in the manufacture of state of the art semiconductive devices there are numerous processes, such as electrochemical chemical polishing and some etching, that depend on substantial uniformity over large areas to provide uniform results over the area processed. With such processes, there tends to be a non-uniformity area effect that causes a difference in results with respect to complete squares close to the wafer periphery because of the discontinuity such periphery provides.
To counter these various edge effects, the step and repeat process is often extended to print squares that extend beyond the area free of such effects, even though it is known that these squares will not provide functional chips. Printing these non-functional squares extends the overall exposure time for printing of a wafer and so decreases the throughput of a given exposure machine. Because of the high cost of the exposure machines, this can be a significant extra expense factor in the manufacture of integrated circuits.
It is also a fact that many integrated circuit devices do not utilize the full exposure field of the equipment available. In particular some scanning equipment, such as deep-ultra violet printers, offer large exposure fields for printing chip squares that often cannot be fully used because the final product made has to fit closely the exposure field of tools, used for other processes in the manufacture, that have much smaller exposure fields.
Because of this, printing equipment now available typically includes provision for controlling the field exposure area of the step pattern or square by providing an exposure tool that controls the size of the area printed. This includes use of movable blades that can be adjusted to set the effective field exposure area of the printing reticle just to that needed for the square being printed. This setting generally remains fixed during the printing of a wafer.
The present invention employs a patterning strategy that better copes with the wafer edge problem described by use of the extra exposure area available in such printing equipment.